Preload useful templates such as header, textinfo and author in respective namespaces.
popups, with revision patrolling capabilities
Colour the page background and text boxes in edit mode to lower contrast and reduce eye-strain.
Add a print/export link to download pages as EPUB files using the WSexport tool.
Pages I can validate. Border colour: Red page can be proofread, green nothing to do, orange checking in progress. Use from an Index: page (see Help:Proofread)
Add a "create book" sidebar link which opens a Wikisource-specific book creator, replacing the PediaPress link (documentation).
Upon creation of an Index: page enables addition of metadata from the file at Commons. More information at Help:Proofread